The idea that links to foreign keys (IDs) should be in the format of a URL is an interesting idea. In practice, I don't think it's a good idea since it makes assumptions about what API consumers want to do with the data -- it assumes they want to fetch it, when there could be several other actions they could take.

Including the link URL as a template in a separate field is a potential possibility, but (a) it will add a ton of duplicated data to collections results, (b) it relies on your consumer parsing and inserting the field into their call every time, which if they care about the efficiency of their own code they're probably not going to do, so you don't get any added flexibility.